Illuminating devices for linearly illuminating a flat object, in particular a bank note, have to fulfill a plurality of conditions in order to be able to meet the demands made on them. These demands comprise, among other things, an illumination as homogeneous as possible with a high light intensity and at the same time low amount of energy used in order to avoid unnecessary heat build-up. In addition, for example for the use in bank note processing machines, it is decisive that the illumination of the object to be measured, i.e. of the bank note that is transported past the illuminating device with a high speed, takes place in a fashion such that there is no or only a low dependence on the distance of the object to be measured to the illuminating device, since the distance may vary due to the transportation of the bank note.
Illuminating devices for linearly illuminating a flat object, in particular a bank note, meeting these requirements are known. Such an illuminating device is known for example from DE 100 00 029 A1. This illuminating device contains a mirror arrangement, which in cross section is disposed symmetrically to an optical axis and consists of several mirror segments directly adjacent to each other.
But it has turned out that the manufacturing of the mirror segments is elaborate, since the mirror segments have a non-linear curve shape, e.g. circular, elliptic, hyperbola-shaped etc. This curve shape makes great demands on the manufacturing of the mirror segments, which usually is effected by milling the mirror segments out of a full metallic material, e.g. aluminum, and a subsequent polishing. Moreover, the assembling of the curved mirror segments as to form the illuminating device is difficult, since the non-linear mirror segments have to be fitted together in an exactly adjusted fashion in order to obtain the desired illumination properties.